extract_metric: started to implement feature to output metric per slice
extract_metric: started to implement feature to output metric per slice


Former-commit-id: d93d1d91791609deb7f45988abc49ab834077d02
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox